1. Field of the Invention
The present invention relates to filter devices preferably for use in duplexers or the like, for example.
2. Description of the Related Art
Various filter devices including a plurality of filters including different pass bands have been proposed. WO2007/145049 A1 discloses a duplexer for a cellular phone. According to the duplexer disclosed in WO2007/145049 A1, a transmission filter chip and a reception filter chip are mounted on a substrate. The transmission filter and the reception filter are connected to an antenna terminal provided on the substrate by a conductor line. A surface acoustic wave resonator is connected to a portion of the conductor line for connecting the filters that is located between the antenna terminal and the reception filter. The surface acoustic wave resonator includes a single interdigital transducer electrode including a single input terminal and a single output terminal.
Conventional duplexers such as that disclosed in WO2007/145049 A1 have had a problem of insufficient isolation characteristics between the transmission filter and the reception filter. The isolation can be improved by disposing the transmission filter chip and the reception filter chip farther away from each other. However, doing so increases the length of wiring between the transmission filter and a common connection point connected to the antenna terminal, or between the reception filter and the common connection point, which in turn increases the electrical length produced by the wiring in a high-frequency electric circuit. Accordingly, the optimum impedance matching between the transmission filter and the reception filter is not achieved. Such impedance mismatching is problematic in that it causes an increase in insertion loss.
Increasing the impedance of a resonator in the transmission filter or the reception filter is conceivable for correcting the impedance mismatching. However, the insertion loss increases in such a case as well.